The main source of the energy for 150 years has been fossil fuels. The United States and virtually every other country in the world depend almost exclusively on fossil-fuel-powered transportation. Planes, trains, automobiles, and other engine-powered devices operate by burning petroleum products such as gasoline and diesel fuel. Fossil fuel, however, is a finite resource. According to some projections, its sources will begin to decline in rate of delivery as early as 2010. Currently, the loss of a reliable supply of fossil fuel would have a devastating effect on the whole of western society. For example, people would not be able to travel to work, factories would not be able to transport their products, and emergency services could not be delivered.
Additionally, petroleum fuels must be harvested, stored, processed, and transported. These steps have led to accidents that severely damage the earth's environment. Even minor oil spills, which happen rather frequently, are deadly to wildlife, detrimental to human health, costly, and difficult to clean. Petroleum fuels emit polluting by-products, such carbon dioxide (CO2) and carbon monoxide (CO), nitrogen oxides (NOx), the main source of urban smog, and unburned hydrocarbons, the main source of urban ozone. All of these chemicals have been medically proven to be detrimental to human health. In big cities and other largely populated areas, poor air quality can have a profoundly accelerated damaging effect on human health.
There is consensus that the search for alternative clean and renewable energy should be a prerogative in the near future (Arunachalam, V. S.; Fleischer, E. L. MRS Bulletin 2008, 33, 264). Engineers and scientists have been working hard trying to find other sources of energy that they can use to replace gasoline. The world has many renewable resources such as sunlight, wind, rain, tides and geothermal heat which is naturally replenished. One of the many options is to use hydrogen as a fuel (Crabtree, G. W.; Dresselhaus, M. S. MRS Bulletin 2008, 33, 421). Of particular interest is hydrogen fueled automobiles.
Hydrogen, provided it is produced using clean and renewable energy sources, such as solar energy, can either be combusted in an internal combustion engine or used in a fuel cell (Fontes, E.; Nilsson, E. Industrial Physicist 2001, 7, 14; Stefanakos, E. K.; Goswami, D. Y.; Srinivasan, S. S.; Wolan, J. T. Hydrogen Energy. In Environmentally Conscious Alternative Energy Production; Kutz, Myer, Eds.; John Wiley & Sons, Inc., 2007; pp 165) to produce energy free of any pollutant by-products, producing solely energy and water. Though there are many barriers towards realizing a hydrogen economy, one of the biggest challenges is to find a safe and efficient means of storing the hydrogen for use in mobile applications (Satyapal, S.; Petrovic, J.; Thomas, G. Scientific American 2007, 296, 80). Many of the major automobile manufacturers, including GM, Chrysler, Ford, and Toyota, are already involved in research and development, investing millions of dollars to find an optimum hydrogen storage system for fuel cell car range of 300 miles. However, widespread use of hydrogen has been limited due to devices with adequate storage capacity, cost, weight, and environmental safety for fuel cell based vehicular applications.
Current options include storing hydrogen in its liquid form or as a compressed gas. Both methods require a large amount of energy and can pose serious safety risks. Therefore, there is a push to find a material to chemically store hydrogen using, for example, metal hydrides (Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353) or complex hydrides (Grochala, W.; Edwards, P. P. Chemical Reviews 2004, 104, 1283).
There are, however, many challenges that these materials must overcome. Specifically, these are to have fast kinetics, a high capacity, e.g. more than 6 wt. % hydrogen, and to be reusable for at least 1000 cycles (Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. Catalysis Today 2007, 120, 246). Advanced complex hydrides that are light weight, low cost and have high hydrogen density are essential for on-board vehicular storage (Biniwale, R. B.; Rayalu, S.; Devotta, S.; Ichikawa, M. International Journal of Hydrogen Energy 2008, 33, 360; David, E. Journal of Materials Processing Technology 2005, 162-163, 169; Guo, Z. X.; Shang, C.; Aguey-Zinsou, K. F. Journal of the European Ceramic Society 2008, 28, 1467; Nijkamp, M. G.; Raaymakers, J. E. M. J.; van Dillen, A. J.; de Jong, K. P. Applied Physics A: Materials Science & Processing 2001, 72, 619; Principi, G.; Agresti, F.; Maddalena, A.; Lo Russo, S. Energy, In Press, Corrected Proof; Ross, D. K. Vacuum 2006, 80, 1084; Zhou, L. Renewable and Sustainable Energy Reviews 2005, 9, 395; Züttel, A. Materials Today 2003, 6, 24). Some of the complex hydrides such as catalyst doped alanates, (Ahluwalia, R. K. International Journal of Hydrogen Energy 2007, 32, 1251; Eigen, N.; Gosch, F.; Dornheim, M.; Klassen, T.; Bormann, R. Journal of Alloys and Compounds 2008, 465, 310; Haiduc, A. G.; Stil, H. A.; Schwarz, M. A.; Paulus, P.; Geerlings, J. J. C. Journal of Alloys and Compounds 2005, 393, 252; Sterlin Leo Hudson, M.; Pukazhselvan, D.; Irene Sheeja, G.; Srivastava, O. N. International Journal of Hydrogen Energy 2007, 32, 4933; Zheng, X.; Qu, X.; Humail, I. S.; Li, P.; Wang, G. International Journal of Hydrogen Energy 2007, 32, 1141; Züttel, A.; Wenger, P.; Sudan, P.; Mauron, P.; Orimo, S.-i. Materials Science and Engineering B 2004, 108, 9) alanes, (Walters, R. T.; Scogin, J. H. Journal of Alloys and Compounds 2004, 379, 135) amide, (Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Journal of Physical Chemistry B 2003, 107, 10967) borohydrides, (Vajo, J. J.; Skeith, S. L.; Mertens, F. J. Phys. Chem. 2005, 109, 3719) magnesium based hydrides, (Dornheim, M.; Doppiu, S.; Barkhordarian, G.; Boesenberg, U.; Klassen, T.; Gutfleisch, O.; Bormann, R. Scripta Materialia 2007, 56, 841) and mixed complex hydrides (Nakamori, Y.; Ninomiya, A.; Kitahara, G.; Aoki, M.; Noritake, T.; Miwa, K.; Kojima, Y.; Orimo, S. Journal of Power Sources 2006, 155, 447) have been recently reported with improved hydrogen storage characteristics.
Hydrogen storage technology is essential for any hydrogen-based transportation system. The development of improved hydrogen storage materials will solve a major storage issue, which, at present, is an impediment for a future hydrogen based economy. This hydrogen storage materials development technology will make the use of hydrogen fuel cells feasible as a long term solution for transportation. This will also provide a solution to various issues related to the present fossil fuels (gasoline, coal etc.); in particular, it will (i) protect the earth's atmosphere from the greenhouse gas emissions, (ii) provide an alternative clean fuel to replace the current depleting gasoline, (iii) provide energy security and (iv) offer pollution free living based on zero-emission vehicular transportation for healthy living.